Enhancement of bivalve production capacity in a landlocked heliothermic marine basin

A landlocked heliothermic marine basin (Norwegian oyster-poll) used as a food production system for a bivalve upwelling nursery was manipulated in order to enhance bivalve production. The manipulations included artificial fertilization, reduction in brackish surface layer, increased supply of water to the bivalve nursery, increased turbulence in the poll by pumping of water and renewal of the poll water. Effects of the manipulations were investigated in a full-scale field experiment comparing data from two monitoring programmes in 1989 and 1990. Manipulations were performed in 1990, but only traditional management was carried out in 1989, which was considered as the control. The bivalve production of 6 t live weight represented a six-fold increase caused by the manipulations. The higher production was mainly a result of the enhanced availability of nutrients which increased primary production and enhanced availability of food for the cultivated bivalves. The manipulations drastically reduced hydrographical stratification and produced temperatures of 18-21^°C. Increased oxygen concentration and pH provided improved water quality for the cultured bivalves. Diatoms became dominant and the dinoflagellates were reduced. The manipulations could be a general aid to the management of polls and similar systems in their application as food production systems in aquaculture.